Installing MongoDB 1.8.1, in my case as a developer database, is easy. This blog post just itemises all the steps so that you can pretty much blindly follow along. I'll probably use these steps myself as I seem to be doing this regurlarly ;-)

Download the 64bit Linux binaries from here and unzip the contents to /usr/local.

The first "ln -s" above sets up a handy symbolic link between the versioned mongodb folder and its unversioned counterpart. When 10Gen release updates, say version 1.8.2, all you need to do is download, unzip, and link the '1.8.2 mongodb folder' to the unversioned folder and 'hey presto' everything should just work.

To get an init script working cleanly with this setup, download mine from my Github 'dotfiles' repo. Please note - my init script enables journaling and the REST interface (on line 51).

Also, you need to activate your MongoDB service's init script by adding it to your system's run-level configuration. That way the service will startup during the boot sequence and stop nicely during the OS' shutdown procedure.

sudo update-rc.d mongod defaults

Lastly to launch MongoDB

/etc/init.d/mongod start

Good luck!

UPDATE: Since April 6 Ubuntu now has prefabbed packages containing MongoDB 1.8.1, maintained by 10Gen. See the instruction below.